I had a similar problem several years ago with my '86 GL.  The four 10mm
nuts underneath the shifter had come loose and one or two were gone.  The
shifter is bolted down through the floor and onto the housing that supports
the front part of the shift rod.  (If I remember correctly.)  There was
enough slop that it would not change gears.  I was able to get home in
second or third gear.  I picked up new nuts at the hardware store.  Drop
your spare and check it out.  Clean and lubricate the shifter while you are
in there.  Good luck.

                                                   Jay
                                                   '85 Westy